This fight is soloable, provided you kite the troll and avoid the lava hot spots in the cavern.

When you reach the entrance to the treasure chest cave, after you kill the veteran destroyer troll, do not rush into the cavern. A Giant Destroyer Troll is waiting there, and its treasure chest (the jumping puzzle chest) will only open after you defeat it. The troll is slow to hit, doesn't do excessive melee damage (about 900-1000), but buffs itself with 5 stacks of Might periodically and ignites the floor at several places which you should avoid (this lava is the main source of damage). You can easily kite it in a circle around a large rock near the entrance, which makes it more difficult for it to come too close to you. The troll periodically pauses to summon a projectile-reflecting shield, making it possible to use melee skills at those times, but having a ranged weapon as well is helpful. The fight can be sped up considerably by kiting the troll over the lava fountains, as it also take heavy damage from the fountains.

There is a chance that the Giant Destroyer Troll will spawn at The Iron Veil frozen lake. If this occurs, there is no way to kill it because it will be permanently invulnerable. If this happens, let the troll reset and it will be killable again.